Move internal functions out of libnetd_resolv's public headers

This is yet another preparatory change before enableing the ABI checker.
Internal functions are now declared in non-public headers paired with
the source files where they are declared.

A followup change could further split resolv_private.h.

Bug: 133117690
Test: No functionality changes, just run atest.
Change-Id: I66ab2f9856e7faa82c74a8ef3fbdeef579577c64